Labour Department programme director Bertha Cheng Wai-yue said she was aware that job-hopping was growing popular among young school leavers.

'They work for a short period of time to make some savings before they quit,' Ms Cheng said.

'Some young people rely on casual jobs to work three days a week and take another four days off. They may be working as a restaurant worker for this month but shift to become a salesperson the next. But companies usually determine promotions and increments for their staff based on their seniority. Those young people may lose this when they keep shifting their jobs.'

Winnie Ying Fung-sau, principal co-ordinator of the Hong Kong Christian Service, said the phenomenon became noticeable last year.

Such workers are hired for jobs such as sales, couriers, kitchen work, distributing flyers and putting up posters at an hourly rate of HK$10 to HK$20, according to Ms Ying.

There are no official numbers to reflect the phenomenon. But Census and Statistics Department figures show an overall increase in the number of people working part-time, with a big increase in those between 15 and 24.

The rate of workers aged 15 and older engaging in jobs with less than 35 working hours a week has jumped from 9.9 per cent in the first quarter of 2001 to 16.8 per cent in the first quarter of this year.

But the rate of workers between 15 and 24 working less than 35 hours a week has doubled from 10.8 per cent five years ago to 20.8 per cent now.

It also means that 71,000 young people from that age group are working part-time, which represents a leap of 80 per cent from the 39,500 doing so in 2001. Their median income has slipped by 26 per cent, from HK$5,500 in 2001 to HK$4,000 for the first quarter of this year.

Chinese University associate professor of economics Terence Chong Tai-leung said: 'It is not easy to observe the impact now ... But the problem will become serious after 10 or 20 years when they [young people] become the major workforce of our economy - unless they can gear up to upgrade themselves.'

Professor Chong attributed the problem to the change of family structure and values, where most families have only one child who will inherit their parents' assets and have no pressure to find a proper job.

Hong Kong College of Technology president and principal Chan Cheuk-hay said there was an urgent need to change young people's attitudes towards work as the city needed the young workforce to balance out the ageing population - a problem that would become serious in about 10 years.

Dr Chan appealed to the government to introduce more job-related courses in schools to help students better understand different industries and develop a proper attitude to work and career at a younger age.